The prior foreign art (German Auslegeschrift No. 1,766,440, FIG. 6 in connection with FIG. 5) describes a fire-signal device of the above mentioned type, the emitter of the field-effect transistor of the amplifier circuit being connected to the junction of the measuring chamber and a reference chamber, and the substrate of this MOS (metal-oxide silicon) field-effect transistor being connected to a voltage divider which is connected between two capacitors charged with positive and negative voltage amplitudes respectively of the pulsating supply voltage of the signal device. It is the purpose of this circuit connection to make it possible to connect one electrode of the measuring chamber directly to one terminal of the signal device, and to avoid that one of the polarities of the supply voltage could mistakenly trigger the signal device. The resistor of the amplifier circuit with which the field-effect transistor is connected, in this case, is connected to that particular signal PG,3 device terminal to which the measuring chamber is connected; the load resistor of the signal circuit, formed by two ohmic resistors, in turn is connected to that terminal of the signal device to which the reference chamber is connected.
In the known fire-signal device the bistable behaviour, i.e. the maintenance of the conductive state of the signal circuit after triggering, is achieved after the cause of the signal response has been eliminated by providing the signal circuit with an additional transistor for a positive feed-back to the bipolar transistor so that the signal circuit constitutes a bistable flip-flop. The additional transistor of the signal circuit in this case necessitates an increased construction expenditure.
There are also known from German Auslegeschrift No. 1,516,529 and from U.S. Pat. No. 3,728,706, issued Apr. 17, 1973, by W. C. Tipton, ionization fire-signal devices in which the substrate terminal of the field-effect transistor of the amplifier stage is shorted with the source electrode of this field-effect transistor, and in which the signal circuit comprises a thyristor. A thyristor, however, is sensitive to fluctuations in the supply voltage and may trigger an erroneous alarm signal when interferences cause brief voltage increases. This disadvantage is of particular gravity where the fire-signal device is installed in a customary manner within a fire-alarm system in which an important number of signal devices are connected parallel to each other between two conductors of a D.C. signalling line connected to a signal center because along such lines pronounced voltage fluctuations may occur, and it is difficult to protect the signal devices against interferences created thereby.
Other similar ionization fire-signal devices are known from German Offenlegungsschrift No. 2,328,872, and from U.S. Patent No. 3,559,196, issued Jan. 26, 1971, by A. Scheidweiler, in which the resistor of the amplifier circuit is connected to the same signal device terminal to which the reference element is connected, and where the load resistor of the signal circuit is connected to the same signal device terminal to which the measuring chamber is connected; the field-effect transistor of the amplifier circuit, however, does not have a separate substrate terminal. In this case, the terminal of the measuring chamber which faces away from the junction of the measuring chamber and the reference element, is connected with the connecting point of the transistor of the signal circuit and its load resistor so that the measuring chamber is connected only indirectly, by way of the load resistor, to the associated signal device terminal, and the transistor of the signal circuit lies parallel to the series connection of the measuring chamber and the reference element. In this manner, the series connection of the measuring chamber and the reference element is shorted when the transistor of the signal circuit becomes conductive, whereby the potential at the junction of measuring chamber and reference element, by way of a positive feed-back, is shifted beyond the particular value which corresponds to the threshold value of the smoke content of the ambient air necessary to trigger a response of the signal device, so that the signal device has a bistable behaviour. This solution has the disadvantage, however, that none of the electrodes of the measuring chamber can be directly connected to a terminal of the signal device, while it is desirable, in order to effectively shield the measuring chamber from interfering fields and in order to insure a simple maintenance of the signal device, to put the outer electrode which is usually so constructed as to be permeable by air, on a fixed potential, usually the ground potential and, for this purpose, to connect this outer electrode directly to a terminal of the signal device.
Still another ionization fire-signal device similar to the aforementioned type is known (British Pat. No. 1,088,976, FIG. 2), in which a resistor of the amplifier stage is connected to that terminal of the signal device to which the reference element is connected and in which the signal circuit, in addition to the load resistor connected to that same terminal of the signal device, is provided with the series connection of the resistive coils of an indicating device and a relay which are inserted between the main current path of the transistor of the signal circuit and that signal device terminal to which one electrode of the measuring chamber is connected. In this instance, the substrate terminal of the field-effect transistor of the amplifier stage is shorted with its source electrode, and no means are provided by which the transistor of the signal circuit may be maintained in a conductive state when the smoke content decreases below the threshold value.